The rwall command sends a message to all of the users logged into
a specified host. The rwall-server package contains the daemon for
receiving such messages, and is disabled by default on Red Hat Linux
systems (it can be very annoying to keep getting all those messages
when you're trying to play Quake--I mean, trying to get some work done).
Install rwall-server if you'd like the ability to receive messages
from users on remote hosts.